The input of exogenous phosphorus and the source-sink variation of sediments are important reasons for the development of lake eutrophication. It is of great importance to analyse the key processes of phosphorus in and out of the lake and its balanced changes for the environmental management of the lake. Aiming at the variation of phosphorus flux input and output of Lake Waihai of Lake Dianchi, we analysed the phosphorus flux in atmospheric deposition and river input and output of Lake Waihai from 2001 to 2018 by the LOADEST model, combined with the hydrological and water quality observation data and HBV hydrological simulation data, evaluated the variation of total phosphorus source and sink in the lake and its influencing factors. The results showed that: ①The phosphorus flux into Lake Waihai has significant inter-annual variation characteristics during 2001-2018, ranging from 118 t/a to 700 t/a. The average phosphorus flux into Lake Waihai from 2001 to 2018 is 280.61 t/a, of which the river input is 251.97 t/a and the atmospheric deposition input is 28.64 t/a. Among the study of meteorological and hydrological factors, the main influencing factors are the river flow into the lake and the amount of water diversion, followed by precipitation. The total annual phosphorus flux from Lake Waihai is 42.25 t/a, ranging from 8 t/a to 84 t/a. The variation of the flux from the lake is mainly influenced by the river flow into the lake and the amount of water diverted, followed by the temperature factor. On the time scale of year and month, Lake Waihai is an environmental sink of phosphorus, and the average annual retention of phosphorus is 238.36 t/a, but there is a source effect in some periods (2009 and 2018) on the daily scale. For the annual environmental sink problem of phosphorus flux in Lake Waihai, although the water diversion project to reduce water pollution in Lake Waihai is effective in reducing the phosphorus retention rate, it cannot effectively solve the problem of phosphorus enrichment in the lake. The continuous phosphorus sink effect in Lake Waihai may increase the risk to water ecology in the future.